Support for showing symbols on mod-morph behaviours

[ThomasAlban]

I have a custom &bspc_del mod-morph behaviour, which outputs backspace normally, but forward delete when shift is held. It would be really nice if, instead of showing blank like this:

It showed the backspace and delete symbols. So more generally, it would be great if, for mod-morph bevaviours, the UI would show both keypress variants on the key rather than just blank. This would make it easier to see at a glance what the key does.

[nickcoutsos]

It's something I'd like to see as well but as always there are edge cases to consider. Something like &gresc (with bindings <&kp ESC>, <&kp GRAVE>) is trivial as the bindings can be represented with fairly short/simple symbols, but there are also keycodes that have longer names no obvious icon to represent them.

And that's just &kp and related behaviors: if instead that were a bluetooth-related mod morph like <&bt BT_SEL 1>, <&bt BT_DISC 1> to select/clear a specific profile it's more complicated. A mod morph that triggers a macro behavior... you have to just assume the worst.

To do this right I'd probably need to identify clear cases where I can expect all of the binding information to fit in a 1u key, as well as to provide a convenient way to show an expanded form in cases where it it won't. This is a problem that affects other "composing" behaviors so I feel it's worth addressing generally and consistently.